The invention relates in general to control systems for hot water boilers or water heaters, and particularly to control systems for high efficiency condensing boilers or water heaters typically used for domestic hot water that utilize a variable firing rate. The invention will be described in relation to a hot water boiler, but it is to be understood that the invention applies as well to a water heater.
The application of a thermostat to boiler control has traditionally been handled by an electromechanical control that presents a digital (on or off) request for heat to a flame safety controller that would actuate a gas valve and purge system on a gas boiler. With the advent of microprocessor-based controls, many new features allow display and control of thermostat information, e.g., setpoint information and control point status on an annunciator screen.
Flame safety boiler controls directly affect those elements that may cause an unsafe condition. Flame safety controls have very high safety standards and require strict testing and failure analysis, particularly for microprocessor-based controls. The required levels of safety, testing, and failure analysis significantly increase the difficulty and cost of implementing controls that meet these standards. Customization and feature enhancements of flame safety controllers are prohibitively expensive due to the cost of certification and testing. Components of the gas-flame safety controller ignition cycle include safety checks, pre-purge, igniter surface preparation, trial for ignition, gas valve actuation, ignition, and post-purge. Manufacturers of flame safety products typically provide flame safety controllers to an original equipment manufacturer (OEM) for boilers. The OEM then integrates these controls into their boiler designs. Some of the boiler control products also incorporate temperature control sensing and setpoints into the device, but these are usually limited to single standalone boiler devices.
Certain boilers can be designed to utilize a secondary heat exchanger which is designed to accommodate condensation of flue gases and removal of the condensate. Such xe2x80x9ccondensing boilersxe2x80x9d will have an efficiency that is significantly higher than a traditional boiler design having only a single heat exchanger. These xe2x80x9ccondensingxe2x80x9d designs typically require that a portion of the water leaving the primary heat exchanger can be routed or fed back to the entering side of the primary heat exchanger. This feedback loop of hot water is to ensure that the water temperature to the main heat exchanger does not go below the condensing temperature of the waste combustion gas, typically around 135 degrees F. In the past, this feedback loop often included a manually controlled valve that could be fixed in a position to allow a portion of the leaving water to bypass the heating coils or other heating elements that transfer heat to the conditioned building and be recirculated through the primary heat exchanger. Other systems in the past had a temperature sensor located in the inlet of the primary heat exchanger which controlled a valve that allowed more or less leaving water to be recirculated, depending on the temperature at the sensor.
New gas valve technologies have evolved that will automatically adjust the boiler combustion air-to-fuel-ratio based on the air pressure of firing rate combustion rate. With the new gas valve technologies, the addition of a variable frequency drive (VFD) allows for xe2x80x9cmodulatingxe2x80x9d or controlling the firing rate of the boilers from a low firing rate to a high firing rate as a function of the demand for heat. In addition, VFD allows purging of the combustion chamber when as is not intended to be present.
Since the boiler is high efficiency, condensation of flue gases is expected to occur in the secondary heat exchanger. If the water temperature at the bypass temperature goes below the condensation temperature for the flue gas, say at 135 degrees F, it would be possible to have condensation at the primary heat exchanger, which is typically considered damaging and undesirable. The bypass valve can be modulated to allow feedback of hot water to warm up the bypass temperature, but if the firing rate is too low on the variable speed fan that generates heat (possibly due to control requirements), the bypass temperature may drop below 135 degrees and allow condensation. The problem just described can occur in situations such as seasonal start up of a boiler system, transition from an unoccupied period to an occupied period where lowered space temperatures were deliberately maintained during the unoccupied period, and specialized low temperature applications. Boiler manufacturers manufacture and distribute high-efficiency boilers without having knowledge about the details of the type of application or end use in which the boiler will be used. Specialized low temperature applications include boiler systems located in low ambient heat spaces, or supplying heating water at lower than normal heating water temperatures. For example, a boiler could be used in a cold climate to heat water for a snow/ice melting system where the water is piped through a concrete driveway or sidewalk. A water heater could be applied to heating a very large reservoir of cold water. Certain low-water-temperature industrial applications are also possible. There is a desire to protect the primary heat exchanger from damage due to condensation at the expense of less than optimum energy efficiency.
Manufacturers of fixed firing rate systems and single stage boilers have attempted to solve this problem by having complex control and interactions with bypass temperature, and flame safety, PID, and other conventional control technologies. Other controls have focused on full on/off firing rates controls and have not dealt effectively with the partially loaded system that is not at full firing rates.
In addition, for reasons of cost effectiveness, the bypass valve is often a butterfly type valve which typically has a percent open versus percent maximum flow rate that is quite nonlinear. A linear relationship between changes in valve position and changes in flow allows stable control to be more easily achieved. A nonlinear relationship or characteristic can lead to control stability problems where a simple control algorithm, for example, proportional control, is used.
Thus, a need exists for a control system for a high efficiency variable firing rate boiler where a bypass valve is controlled to maintain a water temperature entering the primary heat exchanger above the recommended minimum temperature.
The present invention solves these and other needs by providing a control system for a boiler having a variable firing rate, a bypass valve, a primary heat exchanger, and a secondary heat exchanger. A first sensor provides a signal representative of a demand for heat from the boiler for control of the variable firing rate. A bypass valve allows a portion of water leaving the primary heat exchanger to pass through the bypass valve and reenter the primary heat exchanger. A second sensor provides a second signal representative of a temperature of water entering the primary heat exchanger. The position of the bypass valve is controlled to maintain the temperature of water entering the primary heat exchanger above a first temperature. Under conditions when the boiler is operating at a reduced firing rate based on the demand for heat and an increase in firing rate is needed to maintain the temperature of water entering the primary heat exchanger above the first temperature, the variable firing rate is increased.